Apparatus for protecting high frequency piezo-electric crystals



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March 12, 1957 IN VENTOR United States Patent O APPARATUS FOR PRTECTG HIGH FRE- QUEN CY PEZOMELECTRlC CRYSTALS Donald T. Imler, Gettysburg, Pa., assigner, by rncsne signments, to Joiln B. Brady, Washington, D. C.

Application January 31, 1952, Serial No. 269,285 Claims. {CL S10- 9.1)

lvy invention relates broadly to piezo-electric crystal units and more particularly to a method and means for protecting piezo-electric crystal units from the effects of water vapor.

One of the objects of my invention is to provide a construction of high frequency piezo-electric crystal unit having means for protecting the crystal unit against the detrimental effects of water vapor on the operation of high frequency piezo-electric crystals.

Another object of my invention is to provide a construction of insert for the container of a high frequency piezoelectric crystal by which detrimental effects of water vapor of the useful life of the piezo-electric crystal may be eliminated.

Still another object of my invention is to provide means for increasing the eiiiciency and reliability of processing and testing steps in the manufacture of high frequency piezo-electric crystal units by eliminating effects of water vapor on the operation of the high frequency piezo-electric crystal.

A further object of my invention is to provide an arrangement of chemically inert hygroscopic insert for the container of a high frequency crystal for protecting the crystal against detrimental effects of water vapor particles by absorption of any water vapor molecules which may seep into the container.

A still further object of my invention is to provide a method of assembling and sealing high frequency piezoelectric crystal units in containers while protecting the crystals against detrimental influences arising from the presence of water vapor adjacent the surfaces of .the high frequency piezo-electric crystal.

Still another object of my invention resides in a method for stabilizing the operation of high frequency piezo-electric crystals by introducing a chemically inert, naturally hygroscopic material Within the container of a high frequency piezo-electric crystal in such position with respect to the crystal that water vapor molecules are absorbed by molecular attraction within the insert and effect of the water vapor maintained below a level having any appreciable effect upon the operation of the piezo-electric crystal.

Other and further objects of my invention reside in a method of protecting high frequency piezo-electric crystals against defective operation due to the presence of water vapor when subjected to ambient temperatures over a range of approximately -50 degrees centigrade to +90 degrees centigrade, as set forth more fully in the specification hereinafter following by reference to the accompanying drawings, in which:

Figure 1 represents a sheet of chemically inert, naturally hygroscopic material from which the individual inserts are severed for lining the container of a high frequency piezo-electric crystal in accordance with my invention; Fig. 2 is a plan view of one of the severed inserts which is divided from the prepared sheet of Fig. l; Fig. 3 is a top view of the insert represented in Fig. 2; Fig. 4 represents the insert rolled to form to provide a lining for 2,785,321 Patented Mar. 12, 1957 the interior wall of the container of a high frequency piezo-electric crystal; Fig. 5 is a juxtaopposed view of the insert of my invention about to be introduced into the interior of the container for a high frequency piezo-electric crystal; Fig. 6 is a perspective view of the container with the hygroscopic insert in position therein, the containcr and hygroscopic insert being broken away and shown in section; Fig. 7 is a longitudinal vertical sectional view through high frequency piezo-electric crystal holder on line 7-7 of Fig. lO showing the protective insert in position therein around the piezo-electric crystal element within the container; Fig. 8 is a vertical sectional view taken on lines 8-S of Fig. 7; Fig. 9 is a vertical sectional view taken on line 9-9 of Fig. 7; and Fig. l0 is a horizontal sectional taken on line 10-10 of Fig. 7 showing the insert in position within the crystal container.

My invention is directed to an important development in high frequency piezo-electric crystal operation, which l have found so successful, that the number of rejects in the mass production of high frequency piezo-electric crystal units has been very substantially reduced. The demand for reliable high frequency piezo-electric crystal units by the armed services is so great that the method of my invention in eliminating production losses in high frequency piezo-electric crystal units can be readily appreciated. By employing the method of my invention, over-all records in carefully observed production runs of crystal units have shown a 30% better yield through final finishing and testing when using the method of my invention= as compared to similar production runs not using the method of my invention. A particular example of the satisfactory results produced in using the method of my invention in production runs was observed in the manufacture, production and testing of a 13 megacycle piezo-electric crystal unit in which rejects occurred in normal testing operations, whereas when the method of my invention was applied to the same crystal units and the crystal units recheclted in testing, the units passed the test l00%. The method of my invention has been developed after experiencing many failures in operation of high frequency piezo-electric crystal units. I have discovered that the surface of the finished and plated quartz blank is such that, by adsorption, condensation of water vapor takes place. This causes activity dips, that is temporary loss of activity needed to operate the set, or excursions accompanied by frequency instability. ln observing the characteristics of water vapor upon the surface of piezoelectric crystals, l have found that a piezo-electric crystal, when sealed in a container, is ordinarily subject to continued iniluence from water vapor particles. The presence of these water vapor particles continues to impair the piezo-electric activity of the piezo@ electric crystal unit. Upon analysis of conditions existingv in a large number of test units, i have found that water vapor particles show no tendency to settle, but remain evenly dispersed throughout the container of the piezoelectric crystal. Under ordinary conditions there is no appreciable attraction of water vapor molecules for each other, and despite the care in initial finishing, proc. essing and assembling operations, a sufficient quantity of water vapor particles becomes sealed within the container under ordinary production conditions, to adversely influence the operation of the high frequency piezo-electric crystal resulting in the instability and impaired piezoelectric activity to which I have heretofore alluded.

I have discovered that the introduction of a chemically inert, naturaiiy hygroscopic material within the container serves to absorb the water vapor molecules by molecular `attraction and maintain the area adjacent the piezo-electric crystal free of the detrimental effects of water vapor particles upon the high frequency piezo-electric crystal.

Referring to the drawings in detail, reference char- 3 Y acter 1 designates, a sheet .of material which is a nonacid-washed paper, treated with melamine-formaldehyde resin Vfor Vimproved fiber bonding and increased Wet strength. It has normal pll (hot extract method) of 5.5 to 8.0 which, because of the peculiar nature of this paper (absence of buffering impurities), represents es sential neutrality. Fiber content is 100% new cotton rag. The approximate amounts of major constituents are: Alpha-cellulose 9 5 to 9,8% and melamine-formaldehyde resin 1 to 2% Other materials occur only in trace amounts and result in an aggregate Vash content of approximately 60C to V12,00 partsl Vper million. These 4trace constituents in- Chl'de: Iron lOtorlOO parts per million; calcium 100 to 8 0() parts per million; and heavy metals substantially nil.

The :paper represented in Fig. l iS, accurately cut to size by passing the papa1' Sheets through a cutting machine, dividing the main sheet 1 into individual hygroscopic liners shown at 2y in Figs.y 2 and 3. The liner 2 is looped P911 itself in an approximately elliptical contour shown in Fig. 4 at 2', substantially conforming in shape with the interior wall of a piezoelectric crystal container represented at 3 `in Fig. 5. The hygroscopic insert 2 is introduced into the interior of the container 3 with its top edge abutting the linterior of the top of the container and fitted snugly against the interior wall thereof as shown more particularly in Figs. 6 and l0. The insert 2' is retained in the container by frictional adherence with the interior Wall of the container and by engagement with the angularly disposed indent 16 adjacent the top of the container. The indent 16 contains the .small aperture 15 through which the container is evacuated land gas filled. The container thus prepared is ready to be applied over the high frequency piezo-electric crystal mounting as shown more particularly in Figs. 7, 8, 9 Q

and 1.0.

In Figs. 7, 8, 9 and l() the metallic base 4 is in the form of an inverted approximately elliptically shaped pan conforming With the shape of the container 3. The metallic base 4 has a channel shaped upstanding peripheral rim 5 providing means for receiving and securing the downwardly extending skirt of theV container 3. The metallic base 4y is apertured adjacent opposite ends thereof as represented at 4a and 4b. High quality dielectric in,- sulation plastic material 6 is ilowed into the inverted pan 4 during the manufacture of the structure and seeps through the apertures 4a and 4b providing enlarged mounting supports 6a and 6b of insulation material for the downwardly projecting pin terminals 7 and 8. The

plus 7 and 8 extend above the base 4 and serve as supports for the resilient wire members 9 and 10. The resilient Wire members 9 land 10 extend upwardly and termmate in coiled ends 9a and 10a which serve to grip the peripheral edges of the electrodes 11 and 12 and the high frequency quartz disc represented at 14 and establish electrical connection with the opposite face of the high frequency piezo-.electric crystal plate through the electrode lugs or tongues 11a and 12a.V

The 5 of the base which is formed in an inverted channel receives the downwardly depending edge of the container 3 with the hygroscopic insert 2 in position therein. The effect of the hygroscopic insert is noted immediately in the operation of the circuit in which the high frequency piezo-electric crystal is connected for testing, upon placing the container in position in the channel of the rim 5r of the kbase .4. The adsorption of the water vapor particles from the surfaces of the piezoelectric crystal becomes immediately apparent by the st abilized constant frequency operation of the crystaloscillator Ycircuit and the elimination of activity dips therein. Accordingly, freedom of the piezo-electric crystal from the inuence of water vapor particles has an advantage in fthe processing stage as well as with respect to the nished product. Thus in the crystal finishing process the operator platos the .Crystalland then checks `its f reagresser r quency at room temperature and continues this process until the crystal is at the exact required frequency. The crystal is thus plated and then the container is placed over it and this will result in a stability of frequency that is not present if the crystal is exposed to the air or if the container does not contain the insert. Thus the operator gets -a stable reading and is much less likely to nish a crystal that is off frequency at room temperature.

The crysta1 is initially checked with the hygroscopic insert in position, the insert serving to absorb existing water vapor molecules within the container by molecular attraction, and maintain a condition free of water vapor.

n. the production process of my invention, the quarts crystal with the hygroscopic insert is placed in the coner 3, and the Whole is sealed, with the exception of small aperture l5 in the container shown in Figs. 5, nd 7. The unit is then placed in a vacuum chamber ndlthe pressure is reduced to approximately 100 microns. Holding the unit at this pressure for approximately iifteen minutes results in the mechanical removal of a relatively large percentage of water vapor. After completion of the evacuating cycle, the internal pressure is raised to one atmosphere by the admission of dry nitrogen gas through the aperture. The unit is then sealed with the hygroscopic insert in position.

A crystal unit so processed will not show any signs of defective operation due to presence of water vapor when subjected to ambient temperatures of -50 degrees centigrade to +9() degrees centigrade.

Moisture from the human body cannot be tolerated in any degree, since it can be considered an impurity having inferior electrical characteristics and serves only to de base the finished product. Such moisture from the human body is wholly absorbed by the hygroscopic insert within the container thereby eliminating all detrimental effects so far as moisture from the body of the operative is concerned.

The important considerations in both the method and apparatus of my invention are that the sheet of hygroscopic material must have the characteristics of remaining chemically inert Without danger of breaking down or dissolving. Moreover, the chemically inert material which I utilize is essentially chemically pure, as, otherwise, impurities might be added to the crystal unit for impairing the eciency of operation thereof.

The results produced by the protective hygroscopic insert within the piezo-electric crystal unit have been outstanding in performance and reliability characteristics of piezo-electric crystal units thus protected, and although I have described my invention in one of its preferred embodiments, I realize that modifications in ay be made, and I desire that it be understood that no litntaf tions upon my invention are intended other than may be imposed bythe scope ofthe appended claims What I claim as new and desire to secure by Letters Patent of the United States is as follows:

1. in a piezo-electric crystal apparatus, a piezo-electric Crystalv1 mounting means for supporting said piezo-.essais crystal, a container surrounding said piezo-elec crystal and carried. bv said mounting means, a desi s sheet of hygroscopic material shaped to conform with the interior section of said container and forming a liner for the interior of said container and surrounding said piezoelectric crystal and .operating to absorb Water vapor molecules .bvrnolesule attraction Within Said Container, and ineens forming Part of said Container es. c frictional engagement with vsai liner for ma n tawng said liner in position interiorly of said container.

2. In a piezo-electric crystal apparatus, a piezoelectric crystal, mounting means for supporting said piezo-electric crystal, a container surrounding said piezo-electric crystal and carried by said mounting means, a flexible sheet of .hvcrossopc material shaped to conform with .the .interior section of said Container and foro lnsa liner for the interior of said container and surrounding said piezo-electric crystal and operative over an ambient temperature range of approximately -50 degrees centigrade to +90 degrees centigrade for absorbing water vapor molecules by molecular attraction from the surfaces of said piezo-electric crystal, and an indentation formed in said container and establishing frictional engagement with said liner for maintaining said liner in position interiorly of said container.

3. A piezo electric crystal apparatus as set forth in claim 1 in which said sheet of hygroscopic material is a nonacidwashed paper treated with melamineformaldehyde resin.

4. A piezo-electric crystal apparatus as set forth in claim 1 in which said sheet of hygroscopic material contains approximately 95 to 98% alphaacellulose and 1 to 2% melamine-formaldehyde resin.

5. A piezo-electric crystal apparatus as set forth in References Cited in the le of this patent UNITED STATES PATENTS 1,475,548 Hunt Nov. 27, 1923 1,603,468 Jaeger Oct. 19, 1926 2,080,066 Sheppard May 11, 1937 2,423,556 Feibelmann July 8, 1947 2,433,383 Mason Dec. 30, 1947 2,454,244 Wintermute Nov. 16, 1948 2,546,321 Ruggles Mar. 27, 1951 2,597,797 Holmbeck May 20, 1952 

